Cruise controls are now usual in motor vehicles, e.g. cars, trucks and buses. A purpose of a cruise control is to achieve a uniform predetermined vehicle speed, and the cruise control may be of traditional type aiming at constant vehicle speed. This may be achieved by adapting the engine torque to avoid retardation, e.g. by increasing the engine torque uphill. A general purpose for cruise control is to achieve convenient running of the vehicle and also greater comfort for its driver.
A driver of a vehicle with cruise control function usually sets a speed vcc as the speed which he/she wishes the vehicle to maintain until the cruise control function is for any reason deactivated. The cruise control then determines a reference speed vref which it demands from the portion of the vehicle's control system which controls the vehicle's engine, e.g. an engine control unit.
If the cruise control function is of traditional type as above, vref will be equal to the set speed vcc, in which case the vehicle will continually endeavour to maintain the speed vcc set by the driver. A cruise control function of this kind thus causes the vehicle to try to maintain the speed set irrespective of whether it is travelling uphill, downhill or on a horizontal running surface. This means that the vehicle may be accelerated over the crest of a hill, only to be immediately braked on a subsequent downgrade to avoid exceeding the set speed. This method is thus an uneconomic way of running the vehicle, particularly in the case of heavy vehicles.
It is generally the case that apart from the vehicle's procurement cost the main items of expenditure for its routine operation comprise driver pay, repair and maintenance costs and fuel to power the vehicle, and the fuel cost may very greatly affect profitability for an owner of the vehicle, e.g. a haulage company or the like.
For this reason there are cruise control functions which endeavour to modify the running of the vehicle on the basis of knowledge of the road ahead, so that its fuel consumption can be kept as low as possible. This may for example be achieved by means of cruise control functions whereby the reference speed vref can be allowed to deviate from the speed vcc chosen by the driver, on the basis of knowledge of the vehicle's itinerary, in order thereby to run the vehicle in a more fuel-economising way.
An example of such a cruise control is one which uses a so-called “look-ahead” function. A look-ahead cruise control (LACC) is a cruise control which uses knowledge of road sections ahead, i.e. knowledge of the nature of the road ahead of the vehicle, to determine the reference speed signal vref and thus also to modify the vehicle's speed according to variations of the road along which the vehicle travels. Heavy vehicles often have automatically operated gearboxes whereby gear changes are controlled by means of a suitable on-board control system, e.g. a gearbox control system. The vehicle's LACC system assistance may also be largely controlled by one or more of the vehicle's control systems so that the vehicle runs as economically as possible, e.g. by somewhat reducing its speed at the end of an upgrade followed by a downgrade on which the vehicle will be subject to a positive force component provided by gravity in the direction of travel and will therefore accelerate again to desired speed.